The invention relates to a power screwdriver having a drive for a tool spindle having a controller for controlling the drive, and having a monitoring device for monitoring the rotation speed or the torque, which monitoring device is coupled to the controller, in order to switch off the drive when the switch-off criterion is reached, with the controller being programmed such that                (a) the drive is first of all accelerated until the rotation speed has reached a specific first rotation speed;        (b) the rotation speed then falls by at least a specific amount within a specific time increment, or the torque rises by at least a specific amount within a specific time increment, the drive is braked until the rotation speed has reached a specific second rotation speed, which is lower than the first rotation speed.        
A screwdriver such as this is known from EP 1 785 231 A2.
The known screwdriver has a regulating device, by means of which the rotation speed of the motor can be regulated and which reduces the rotation speed when a trigger parameter is reached. In this case, an angular velocity change per unit time is preferably used as the trigger parameter. If it is found that the angular velocity has slowed down, then the rotation speed is reduced, possibly in a plurality of steps, with the intention of ensuring a relatively accurate tightening torque for the screwdriving process. In this case, the aim is for the discrepancy in the tightening torque between hard screwdriving and soft screwdriving to be small. So-called “soft screwdriving” means a screwdriving process in which the torque rises continuously towards the end of the screwdriving process, until the maximum tightening torque is reached. In the case of “hard screwdriving”, the torque is in contrast initially relatively low and rises suddenly and sharply towards the end of the screwdriving process.
In one alternative embodiment of the known screwdriver, the rotation speed is reduced to zero after reaching the trigger parameter, the motor is operated in the opposite rotation direction for a specific time, the rotation direction is then once again reversed, and the screwdriving process is completed at a lower rotation speed than the initial rotation speed.
In the case of the already known screwdriver, although a relatively uniform tightening torque is achieved irrespective of the type of screwdriving process, the total time for completing a screwdriving process is relatively long, particularly in the case of soft screwdriving, since a lower rotation speed is always used at the end, and in some cases is reduced even further. When the rotation direction is reversed, the total time for completing the screwdriving process is increased even further.
DE 10 2008 033 866 A1 discloses a further screwdriver, in which a limiting device is provided in order to limit an output torque, which is produced on the output drive side of the drive train, to a maximum torque value, with the limiting device being designed to operate a current-flow device, which passes current through the drive motor, in a braking mode, by the current-flow device producing a rotating field which brakes the drive motor and is in the opposite sense to the respective rotation direction of the drive motor. Rotation energy in the drive train is taken into account in this case.
This device is intended in particular to make it possible to avoid excessive tightening during hard screwdriving.
Said control for the screwdriver is relatively complicated and actually does not ensure, for any application, that the tightening torque is maintained precisely irrespective of the type of screwdriving process, while at the same time completing the screwdriving process in as short a time as possible.